Columbine High School

Ed Andrieski / Associated Press

A student leaves Columbine High School after a lockout was lifted Thursday. The school, where a shooting left 13 people dead in 1999, went on high security alert this week after receiving a series of threatening phone calls, law enforcement said.

A student leaves Columbine High School after a lockout was lifted Thursday. The school, where a shooting left 13 people dead in 1999, went on high security alert this week after receiving a series of threatening phone calls, law enforcement said. (Ed Andrieski / Associated Press)

A student leaves Columbine High School after a lockout was lifted Thursday. The school, where a shooting left 13 people dead in 1999, went on high security alert this week after receiving a series of threatening phone calls, law enforcement said.